Multi-flow exhaust systems are used for reliably discharging high volume exhaust gas flows with little resistance. High volume exhaust gas flows occur in particular with powerful engines. It is characteristic for multi-flow exhaust systems that exhaust gas from the internal combustion engine and exhaust gas passing through the exhaust system is discharged into the surroundings via at least two tailpipes of the exhaust system.
Regardless of an internal combustion engine's construction (for instance reciprocating piston engine, pistonless rotary engine or free-piston engine), noises are generated as a result of the successively executed strokes (in particular intake and compression of the fuel-air mixture, combustion and discharge of the combusted fuel-air mixture). On the one hand, the noises propagate through the internal combustion engine in the form of solid-borne sound and are emitted on the outside of the internal combustion engine in the form of airborne sound. On the other hand, the noises propagate in the form of airborne sound together with the combusted fuel-air mixture through an exhaust system that is in fluid communication with the internal combustion engine.
These noises are often regarded as being disadvantageous. On the one hand, there are statutory provisions on protection against noise to be observed by manufacturers of vehicles driven by internal combustion engines. These statutory provisions normally specify a maximum allowable sound pressure for an operation of a vehicle. Manufacturers, on the other hand, try to impart a characteristic noise emission to internal combustion engine driven vehicles of their production, with the noise emission fitting the image of the respective manufacturer and being popular with customers. Present-day engines with small displacement often cannot naturally generate such intended characteristic noise.
The noises propagating through the internal combustion engine in the form of solid-borne sound can be muffled quite well and are thus usually no problem as far as protection against noise is concerned. With the increasing use of internal combustion engines having small displacements or even of electric motors, the problem arises that the engine (or motor) noise is often not attractive for users and/or does not fit the image of a vehicle manufacturer.
The noises traveling together with the combusted fuel-air mixture in the form of airborne sound through the exhaust system of the internal combustion engine are reduced by exhaust mufflers located ahead of the exhaust system's discharge opening and downstream of catalytic converters if present. Respective mufflers may for instance work according to the absorption and/or reflection principle. The disadvantage of both operating principles is that they require a comparatively large volume and create a comparatively high resistance to the combusted fuel-air mixture resulting in a drop of the vehicle's overall efficiency and an increased fuel consumption.
For quite some time, so-called anti-noise (anti-sound) systems have been developed as an alternative or supplement to mufflers, which superimpose electro-acoustically generated anti-noise on airborne noise generated by the internal combustion engine and propagated through the exhaust system. Respective systems are for instance known from the following documents: U.S. Pat. Nos. 4,177,874, 5,229,556, 5,233,137, 5,343,533, 5,336,856, 5,432,857, 5,600,106, 5,619,020, EP 0 373 188, EP 0 674 097, EP 0 755 045, EP 0 916 817, EP 1 055 804, EP 1 627 996, DE 197 51 596, DE 10 2006 042 224, DE 10 2008 018 085 and DE 10 2009 031 848.
Respective anti-noise systems typically use a so-called Filtered-X, Least Mean Squares (FxLMS) algorithm trying to turn an error signal measured with an error microphone by outputting acoustic noise with at least one loudspeaker being in fluid communication with the exhaust system down to zero (in the case of noise-cancellation) or to a preset threshold (in the case of influencing noise). For achieving a completely destructive interference between the sound waves propagating through the exhaust system and the anti-noise generated by the loudspeaker, the sound waves originating from the loudspeaker have to match the sound waves propagating through the exhaust system in amplitude and frequency, however, with a relative phase shift of 180 degrees. If the anti-noise sound waves generated at the loudspeaker match the sound waves of the airborne sound propagating through the exhaust system in frequency and have a phase shift of 180 degrees relative thereto, but do not match the sound waves in amplitude, only an attenuation of the sound waves of the airborne sound propagating through the exhaust system is achieved. The anti-noise is calculated separately for each frequency band of the airborne noise propagating through the exhaust pipe using the FxLMS-algorithm by determining a proper frequency and phasing of two sine oscillations being shifted with respect to each other by 90 degrees, and by calculating the required amplitudes for these sine oscillations. The objective of anti-noise systems is that the cancellation or influencing of sound is audible and measurable at least outside of, but, as the case may be, also inside the exhaust system. The term “anti-noise” used in this document serves for distinguishing sound output by the at least one loudspeaker of an anti-noise system against airborne sound propagating through the exhaust system as a result of the successively executed strokes of the combustion engine. In itself, anti-noise is simple airborne sound. It is pointed out that the present document is not limited to a use of an FxLMS algorithm.
An exhaust system involving an anti-noise-system according to the prior art is explained below with reference to FIGS. 1 and 2.
An exhaust system featuring an anti-noise-system 1 comprises a sound generator 2 in the form of a soundproofed housing which contains a loudspeaker 3 and which is connected to an exhaust system 6 in the region of a tailpipe 4.
The tailpipe 4 includes a discharge opening 5 for discharging exhaust gas passing through the exhaust system to the environment.
An error microphone 7 in the form of a pressure sensor is provided at the tailpipe 4. The error microphone 7 measures the pressure variations and thus the noise inside the tailpipe 4 in a section downstream of a region providing the fluid connection between the exhaust system 6 and the sound generator 2. The term “downstream” hereby relates to the direction of the exhaust gas flow. The direction of the exhaust gas flow is indicated by arrows in FIG. 2.
The loudspeaker 3 and the error microphone 7 are electrically connected to an (anti-noise) controller 8. Further, the controller 8 is connected to an engine control unit 9 of an internal combustion engine 10 via a CAN data bus.
Using a Filtered-x Least Means Squares (FxLMS) algorithm, the anti-noise controller 8 calculates a digital control signal for a loudspeaker 3 based on the noise measured with the error microphone 7 and based on the operating parameters of the combustion engine 10 received via the CAN data bus, whereby the digital control signal enables a substantial silencing of the noise propagating through the interiors of the exhaust system 6 by application of anti-noise and is provided to loudspeaker 3.
It is a disadvantage of already known systems for influencing exhaust noise that these systems are not designed for multi-flow exhaust systems.